Process of heat treating magnetic material



Dec. 13, 1938. T. D. YENSEN .ET-A1. 2,140,374 PROCESS OF HEAT TREATING MAGNETIC MATERIAL Filed May 29, 1956 Patented Dee. 13,1938

UNITED- STATES PATENT ort-ICE PROCESS 0F HEAT TREATING MAGNETIC VIATERIAL Trygve D. Yensen and Stephen L. Burgwin, Forest Hills, Pa., assgnors to Westinghouse Electric & Manufacturing Company, IEast Pittsburgh, Ba., a corporation of Pennsylvania Application May 29, 1936, Serial No. 82,478

e 3 Claims.-

stantially lowering the content of the carbonL impurity which is normally present in the alloy. 'Ihese heat treatments, however, are not successful for completely removing the carbon impurity but function to reduce the carbon content to about .005% by weight or somewhat higher.

When the silicon iron alloy containing the carbon impurity is employed in the industry, it is often subjected to operating temperatures of between 50 C. and 100 C. or higher. Under these conditions it is found that the alloy so ages because of the precipitation of the carbon impurity at the operating temperature that the magnetic properties are greatly impaired.

An object of thisinvention is4 to provide a heat treatment for\ silicon iron alloys whereby the ellect of ageing o/f the alloy is substantially avoided.

A more specic object of this invention is to provide a heat treatment for causing a precipitation of the carbon in silicon iron alloys withoutthe introduction of ageing strains in the alloy.

This invention will be better understood by reference to the following description when taken in conJunction with the accompanying drawing, in which:

Figure 1 is a representation of the equilibrium diagram of carbon in a silicon iron alloy,

Fig. 2 is an arbitrary `chart illustrating the hysteresis losses in silicon iron alloys which have been subjected to an annealing heat treatment to lower the carbon impurity content, the increase in the hysteresis losses for the same alloy when subjected to ageing, the hysteresis losses encountered in the same alloylwhen heat treated in accordancewith the teachings of this invention and the increase of the hysteresis losses in the same alloy when aged.

The expression ageing treatment as employed in' this application is intended to mean the treatment whereby normally annealed alloys are subjected to a' heating which would normally cause a precipitation of carbon from solution and introduce strains in the alloy. In the specic examples given, an ageing treatment at C. is employed in order to obtain an accelerated precipitatlon of the carbon. 'I'he strains which are set up in a silicon alloy cooled from a high temperature to atmospheric in au ageing treatment (ci. 14a-21.5)

cause an increase in the hysteresis losses when employed in electrical apparatus and an impair-` ment of its magnetic properties. This increase in losses is generally known as "ageing or the effect of ageing. l g

In accordance with the teachings of this invention, silicon iron alloys containing up to 4.75% silicon may be subjected to a suitable heat treatment which does not cause an introduction of strains such as are ordinarily caused by the ageing treatments heretofore employed.

Through experiments, it was found that the carbon content present in the alloys as an impurity precipitates as ilne particles which are dispersed throughout the alloy. These precipitations introduce strains in the alloy which materially decrease its magnetic properties. This phenomenon is generally known as ageing by those skilled in .g the art. f In practice it is desired to so reduce the carbon 20 content to a minimum that the maximum magnetic characteristics may be developed. A reduction of the carbon contentto about .005% by weight may be effected by heating the alloy at a temperature of between 1000 .C. and 1300 C. 25 in a reducing atmosphere. 'I'his heat treatment substantially lowers the carbon impurity content and releases the strains normally set up in the alloy during the working of it. Modiilcations of the annealing heat treatment such as control of 30 the moisture content, and the ilow of the reducing gas, maybe employed in eiecting the reduction of the carbonv impurity content.

and introduce strains in the alloy. The resulting 45 l strains greatly increase thev hysteresis losses of the1 alloy and impairs its use as magnetic materi In order to avoid the effectl of ageing causedv by the precipitation of the carbon in the alloy, 50 namely, the -setting up of strains inthe material which cause an increase in the hysteresis losses, the silicon iron alloys which have been annealed l to reduce the carbon contentto a minimum may be subjected to a further heat treatment at a ity temperature of between, 300 C. and 000 C. for a period or time of from two to one hundred hours. Ylllirough experiments, it is found that this heat treatment produces an alloy which is substantially free from the effect of ageing.

its a possible explanation of this phenomenon it is believed that the heating of the alloy at a temperatiue of between 300 and 600 C. yfor a time oi from 2 to 100 hours causes a precipitation of the carbon in excess of the solid solubility con tent at the heating temperature in such a manner that strains are not set up in the alloy. `lihere the lower heating temperature is employed, a longer period of the heat treatment is necessary to obtain a complete precipitation ci the carbon content in excess ci the solid solubility content at the heating temperature. With the higher neating temperature the desired precipitation is eifected in the shorter period of time.

Through experiments, it was found that heating the annealed alloy at a temperature of loen tween 400 C. and 500 C. in hydrogen ior a period of time oi about 6 hours is quite effective' in causing the precipitation of the carbon impurwithout introducing strains, although a longer period' of heating may be necessary. When the alloys have been subjected to this heat treatment and then subjectedto an ageing temperature ci l C., the increase in the hysteresis i. losses caused by the ageing treatment is so slight In this connecthe as to be practically negligible. tion reference ymay be had -to Fig. 2 oi drawing.

In the figure, the representations i0, l2, l0' and it to the left of the ordinate are a measure oi. the hysteresis-losses encountered in silicon iron alloy samples 0, II, i3 and l5 which have a 3% silicon'content and which have been subjected to the annealing treatment for reducing the carbon impurities. The representations I0, 20, 22 and 25- shown in dotted lines to the right of the ordinate are a measure of the increase in the hysteresis losses of theannealed samples il, Il, I3 and ili respectively, when subjected to an ageing temperature of 100 C. for 600 hours. The int crease in the losses for the diiferenosamples when aged are more clearly shown in the Jfollow- 'ing table in which it is seen that the total losses of the alloys are of such a Value as to impair the magnetic characteristics of the alloys.

Hysteresz's loss in ergs/ca/cycle for F132111000 gousses As anincrease Total Sample nealedet when aged loss 'after 1100 C. at 100 C. ageing 30o 21o 51o 280 210 490 29o 21o v 50o 295 21o 505` When the samples 9, II, I3 and I5 are subjected to the heat treatment ofbetween ti00 C. and 500 C. for a period of 6 hours, the initial hysteresis losses encountered as shown in the drawing by the representations 26, 28. 30 and 32 are comparable to the initial hysteresis losses found in the annealed samples, as indicated by the representations vII), I2, I4 and I6, respectively. The samples 9, I,I and I3 were subjected to a heating temperature of 4009 C., 450 C., 500 (E. respectively, in hydrogen, for a period of 6 of 90 hours.-

m order to illustrate the eEect of the heat treatment in avoiding the introduction of ageing strains in the alloy and consequently avoiding an increase in the hysteresis losses, the heat treated samples 0, il, and iii may be subjected to an ageing treatment at l00 C. for 600 hours. The

increase in the hysteresis losses of the alloys,v

when agedLare shown by the representations 3e, andY l0 for the samples 0, III, i3 and i5, respectively. This improvement maybe better understood by reference to the following table in which the increase in the hysteresis losses is clearly set forth. Y i

Hysteeszs loss in ergs/oo /cycle for B=10,000

fr, gousses Increase `Total As annealed at l100 n Sample C. and heated at-f vgthod lsegr 9 315 (400 C/G l1rS.)- 3 318 11 33001.50c C/ hrS.)... V3 333 13 340 (500 C/ hrS.). 25 365 l 295 (500 C/Qll hrs.) 5 300 in examination of the total hysteresis losses after ageing at 100 C. for the samples 0, ii, Lt`

and C5 treated by the process hereinbefore described when compared with the total losses Y neutral or reducing atmosphere for a period of time of about 6 hours. The annealed alloy which has been freed from shearing strains may then be subjected to a heating temperature of from 400 C. to 500 C. in hydrogenl for a period of time of from 2 to 100x hours to cause the precipi- I tation of the carbon impurity in excess of the solid solubility content at the heating temperature without introducing strains caused by the precipitation, as explained hereinbefore.

In Fig. 2 of the drawing, representation has been made of the results obtained by subjecting samples which have been annealed at l200 C. to the further heat treatment comprising heating at 700 C. in hydrogen for 6 hours followed by a separate heating at a temperature of 400 C. in hydrogen for a period 'of 6 hours. In the gure,

50, 52, 5d and 50 are representations of the hys-A teresis losses encountered in samples 42, 44, 4t"

and 40, treatment.

When these samples are subjected to ageing at 100 C. for a period of 600 hours, the hysteresis losses increaseas indicated by the representations 58, 00, E2 and 64, respectively, shown in dotted lines to the right of the ordinate. After the samples 42, E4, 40 and 48 are sheared to the respectively, after the annealing desired size and then subjected to the further.

heat treatment at a temperature of 700 C. followed by a separate heat treatment at 400 C., as described, the initial hysteresis losses in the heat treated samples, as indicated by the representations 6E, 60, 10 and 12, respectively, are found to be comparable to the initial hysteresis losses of the annealed alloy.

When subjected to the ageing treatment, it is Hysteresis loss in ergs/cc/cycle for B=10,000

ga'ILSSGS Increase Total As annealed Sample f when aged loss when at 12000 C' at 100 C. aged 410Y 530 940 490 820 l3l0 490 240 730 650 100 750 As anioealCe-d at l2 Increase Total Sample adld when aged loss when followed b'y at 100 C. aged A modification of this 'heat treatment is to vsubject the alloy to a continuous heat treatment after it has been sheared.. 'I'his may be accomplished by heating the sheared and annealed alloy at a temperature of between 700 C. and 800 C. in hydrogen fora period of about 6 h ours and then lowering the temperature to between 400 C.

' and 500 C. and heating it at that temperature for a period of about 6 hours.

A specic heat treatment embodying this modification which has been found desirable, is to anneal the alloy at a temperature of between l000 C. and 1300" C. in a reducing atmosphere to lower the impurity content, shear the annealed alloy to the desired size and then to heat the sheared alloy at a temperature of about 700 C. in hydrogen for a period of 6 hours to release the shearing strains at the ends of which time the temperature is lowered to 450 C. and held at that temperature for a period of 6 hours to cause the desired precipitation of carbon substantially without strains after which the alloy is cooled to room temperature. It is found that this heat treatment eiectively removes the shearing strains and I substantially avoids the effect of ageing.

A study of photomicrographs of the alloys which are subjected to the heat treatments hereinbefore described reveals that the ageing strains which may be found in the alloys subjected to the ageing heat treatment after the annealing therel of are not present in the alloys treated in accordance with the process'of this invention. An analysis of the alloys treated proves that the carbon content has not been substantially reduced by the heat treatment hereinbefore described, and a possible explanation of'the improvement in the conditioning of the alloy is that a precipitation of the carbon impurity content is eiected by this heat treatment without the retention of strains. Since, according to this explanation, only a small portion of the carbon.

content remains in solid solution in the alloy after subjecting it to the heating temperature, it is evident that upon subjecting the alloy to the ageing treatment there will be obtainedl only a negligible precipitation of the carbon remaining in solid solution in the alloy. Since the ageing is negligible, it is evident that the magnetic characteristics of the alloy will not be substantially impaired.

' Although this invention has been described with reference to a .particular process, it is evident that other and various modifications thereof are possible. It is, therefore, not to be restricted except insofaras is necessitated by the prior art and the scope of the appended claims.

We claim as our invention:

l. In the process of heat treating silicon iron alloys which have been heat treated to release working strains and lower the carbon impurity content in excess of the solid solubility content of carbon in the alloy at a temperature of 300 C., in combination, heating the alloy in a reducing atmosphere to avtemperature ranging from 400 C. to 500 C. for a period of time of from 6 to 100 hours to stabilize the magnetic characteristics of the alloy without effecting a substantial increase in the hysteresis loss of the alloy whereby the f alloy may be subjected to a testing temperature shearing the annealed alloy into sheets,. subjecting the sheared alloy sheets to a temperature ranging from 600 C. to 800 C. in a hydrogen atmosphere to release the shearing strains, cooling the alloy sheets, and subjecting the alloy sheets which are freed from shearing strains to a temperature ranging from 400 C. to 500 C. for a period of time of from 2 to 100 hours toy stabilize the magnetic characteristics of the ailoy .f without effecting a substantial increase in its hysteresis loss whereby the alloy may be subjected to a testing temperature of 100 C. for a period of time of 600 hours without effecting an increase in the magnetic ageing of the alloy.

3. In the process of heat treating silicon iron alloys having as an impurity a carbon content in excess of the solid solubility content of carbon in the alloy at a temperature of 300 C., in combination, subjecting the alloy to a heating temperature of between v1000" C. and 1300 C. to anneal it, shearing the annealed alloy into sheets, heating the sheared alloy sheets at a temperature of between 600 C.and`800 C. to release the shearing strains, reducing the heating temperature to a temperature of between 400 C. and 500 C., and heating the alloy sheets at that temperature for a period of time of from 2 to 100 hours to stabilize the magnetic characteristics of the alloy without eecting a substantial increase inthe hysteresis losses whereby the alloy may be subjected to a testing temperatureof 100 C. for a period of time of 600 hours Without eilecting an increase in the magnetic ageing of the alloy.

` TRYGVE D. YENSEN.

STEPHEN L. BURGWIN. 

